As a semiconductor material for semiconductor laser devices (LDs) and light emitting diode devices (LEDs) having emission wavelengths within a wavelength range of ultraviolet to green, gallium nitride semiconductors (GaInAlN) are used. A blue LD using such a gallium nitride semiconductor is described in, for example, Applied Physics Letters, vol. 69, No. 10, p. 1477-1479, and a sectional view of the blue LD is shown in FIG. 19. FIG. 20 is an enlarged view of part E in FIG. 19.
Referring to FIG. 19, reference numeral 101 denotes a sapphire substrate, 102 denotes a GaN buffer layer, 103 denotes an n-GaN contact layer, 104 denotes an n-In0.05Ga0.95N layer, 105 denotes an n-Al0.05Ga0.95N cladding layer, 106 denotes an n-GaN guide layer, 107 denotes a multi-quantum-well structure active layer composed of In0.2Ga0.8N quantum well layers and In0.05Ga0.95N barrier layers, 108 denotes a p-Al0.2Ga0.8N layer, 109 denotes a p-GaN guide layer, 110 denotes a p-Al0.05Ga0.95N cladding layer, 111 denotes a p-GaN contact layer, 112 denotes a p-side electrode, 113 denotes an n-side electrode, and 114 denotes a SiO2 insulating film. In this arrangement, as shown in FIG. 20, the multi-quantum-well structure active layer 107 is composed of five 3 nm thick In0.2Ga0.8N quantum well layers 117 and four 6 nm thick In0.05Ga0.95N barrier layers 118, totally nine layers, where the quantum well layers and the barrier layers are alternately formed.
Also, in Applied Physics Letters, vol. 69, No. 20, p. 3034-3036, there is described a structure that the quantum well structure active layer is composed of alternately stacked three 4 nm thick quantum well layers and two 8 nm thick barrier layers, totally five layers.
Japanese Patent Laid-Open Publication HEI 8-316528 also describes a blue LD using a gallium nitride semiconductor. This prior-art blue LD also uses a multi-quantum-well structure active layer having five or more quantum well layers, as in the case shown in FIGS. 19 and 20.
Meanwhile, a blue LED using a gallium nitride semiconductor is described in, for example, the aforementioned Japanese Patent Laid-Open Publication HEI 8-316528, and a sectional view of the blue LED is shown in FIG. 21. Referring to FIG. 21, reference numeral 121 denotes a sapphire substrate, 122 denotes a GaN buffer layer, 123 denotes an n-GaN contact layer, 124 denotes an n-Al0.3Ga0.7N second cladding layer, 125 denotes an n-In00.1Ga0.99N first cladding layer, 126 denotes a 3 nm thick In0.05Ga0.95N single-quantum-well structure active layer, 127 denotes a p-In0.01Ga0.99N first cladding layer, 128 denotes a p-Al0.3Ga0.7N second cladding layer, 129 denotes a p-GaN contact layer, 130 denotes ap-side electrode, and 131 denotes an n-side electrode. Like this, in blue LEDs using gallium nitride semiconductors, an active layer having only one quantum well layer has been used.
The conventional blue LDs and blue LED described above, however, have had the following problems.
Referring first to the blue LDs, the value of oscillation threshold current is as high as 100 mA or more and so needs to be largely reduced for practical use in information processing for optical disks or the like. Further, if the LD is used for optical disks, in order to prevent data read errors due to noise during data reading, it is necessary to inject a high-frequency current of an about 300 MHz frequency into the LD and modulate an optical output power with the same frequency. In the conventional blue LDs, however, optical output power is not modulated even if a high-frequency current is injected, causing a problem of data read errors.
Referring now to blue LEDs, which indeed have been in practical use, in order to allow blue LEDs to be used for a wider variety of applications including, for example, large color displays capable of displaying bright even at wide angles of visibility, it is desired to realize even higher brightness LEDs by improving optical output power.
Furthermore, conventional gallium nitride LEDs have a problem that as the injection current increases, the peak value of emission wavelengths largely varies. For example, in a gallium nitride blue LED, as the forward current is increased from 0.1 mA to 20 mA, the peak value of emission wavelengths shifts by as much as 7 nm. This is particularly noticeable in LED devices having long emission wavelengths; for example, in a gallium nitride green LED, the peak value of emission wavelengths shifts by as much as 20 nm. When such a device is used in a color display, there would occur a problem that colors of images vary depending on the brightness of the images because of the shift of the peak wavelength.